1. Field of the Invention
The present invention relates to a disk screen classifier, and more particularly to a disk screen classifier for wood chips are classified with respect to the amount of sawdust, fines and oversized material.
2. Description of the Prior Art
Traditionally, the amount of sawdust, fines and oversized material in a batch of wood chips are sampled for each truck load or car load of wood chips at the point of purchase for pulp mills. The sampling and classification provides a quality index which is used as a basis for payment to the wood chip suppliers. A conventional classifier is known in the industry as the Williams Classifier which is a multi-deck shaker screen arrangement having round holes in various screen decks with larger holes at the top and progressively smaller holes at each stage down to the bottom. The sample taken from each truck load or car load is dropped onto the top deck while the screen is operating and the various size particles are segregated and retained on the various screen decks. The classifiers run for approximately 10 minutes, then dismantled and each tray is lifted out of its frame and dumped so that the material on each tray can be weighed. The size fractions are then all expressed as percentages of the total. As will be appreciated, this classification technique is a very laborious and tedious job in that the trays are very heavy and awkward to handle. The industry has therefore been seeking a better method and apparatus for classification of wood chips.
A chip thickness screening program, well known in the art, has developed a disk screen for screening chips according to thickness. This concept is rapidly becoming the new standard in the industry in that it is the thickness of a chip which determines liquor penetration and delignification. With such thickness screens, one may now screen out the overthick material and process the same through chip slicers in that there should be no material fed into a digester which is too thick to be properly pulped. Typical of this type of equipment is the disk screen of U.S. Pat. No. 4,301,930, issued Nov. 24, 1981 and fully incorporated herein by this reference.
During the industry evaluation of the effects of thickness screening, several types of shaker type classifiers were developed which supposedly classified chips according to thickness. Instead of round holes in the various screen decks, the decks were made by spaced steel rods providing gaps therebetween as the desired slot width. These screen decks were initially considered desirable, but they still have the disadvantages of the Williams type classifier. The trays are very heavy and the screen must be dismantled at the end of each classification run and the material picked out of the slots between the rods. Testing of this type of classifier provided and insight to all of the problems which users in the industry found undesirable, such as:
1. The sample which can be classified is very small, usually less than 1/2 cu. ft., or less than 10 lbs. This sample is too small to be representative of the totality of material in an entire railcar or a whole truck bed.
2. The sampling time was long and very laborious. The shaker screen was usually operated for 10 minutes and then dismantled so that each tray could be cleaned and the chips retained on that tray weighed. Generally, the time for classification of 1/2 cu. ft. of material was about 30 minutes.
3. The efficiency of these slot screen classifiers varies between 70% and 90% and is completely inconsistent. Two chips which should pass through the slot of a given width will wedge in the slot and, therefore, be retained on the wrong screen. To use these slot screen classifiers as a device for rating the aforementioned disk screens with respect to screen efficiency, one had to handsort according to thickness almost all of the material on some of the screen decks.
4. The rods and the various screen decks tend to bend slightly during operation and, after a certain time, the slot widths in any screen deck increase or decrease by as much as 2 mm which also contributes to the inefficiency of such classifiers.